Magnets & Magnetic Fields Chapter 21

Magnets Magnets have 2 poles, North and South if broken in half, each half will have both poles at the ends. Like poles repel, unlike poles attract. Hard Magnets- materials that are hard to magnetize and also hard to have lose their magnetism. (ex. Cobalt, nickel) Soft Magnets- materials that are easy to magnetize and also easy to have lose their magnetism. (ex. Iron)

Properties of Soft Magnets Iron can become magnetized by stroking it with a hard permanent magnet. Iron can become magnetized by sitting near a strong permanent magnet for a long time. Iron can lose magnetism by heating, cooling or hammering the iron. Basically disrupting with energy changes.

Magnetic Field Lines Magnetic field lines are similar to electric field lines in the way they are drawn. The north pole is similar to a + charge and a south pole similar to a – charge. The lines go from north to south. However magnetic poles are not charges!

Magnetic Field Direction The direction of the magnetic field is the direction in which the north pole of a compass needle points at that location. Note: This means the head of the magnetic compass needle is a north pole.

Poles of the Earth The Earth’s magnetic field is largely due to the speed of rotation of the Earth. If the compass needle head is a north pole, then how does it point north geographically?

True North Magnetic South S N Magnetic North True South

Magnetic Declination Magnetic declination - angle that the compass is off from true north. varies depending on where you are on Earth If you go past the end of the magnetic poles, the compass needle will turn to point the other way. Since the pole of the magnetic field of the earth is actually in the ground, a 3-D compass will attempt to point straight down at magnetic south (just north of Hudson Bay, in Canada) and straight up at magnetic north.

Magnetic Field Directions Magnetic fields require the use of 3-D vector orientations. The following is a list of symbols and labels for directions. Symbol Direction Plane Right + X Left - X Up + Y Down - Y ∙ Out of plane + Z X Into Plane -Z

Magnetic Domains Electrons spin on their axis causing a small magnetic field. In atoms with many electrons, the electrons tend to pair with an electron with an opposite spin. results in the cancelling of the magnetic field EX: wood and plastics (never become magnetic) Metals usually have unpaired electrons, but not all metals are magnetic. iron, cobalt and nickel have electron spins that do not cancel completely, they are ferromagnetic.

Magnetic Domains (cont.) Opposing electron spins: magnetic field cancels Similar spins magnetic fields add together called domains large group of similarly spinning electrons create a strong magnetic field. North pole spins one way, south pole the other way. Domains can be permanent or temporary. a strong magnetic field near an unmagnetized ferromagnetic material can create domains that can remain when the field is removed permanent magnet

Electromagnetism A wire with current running through it will produce a magnetic field. The direction of the magnetic field is dependent on the direction of the current through the wire. For this we use the Right Hand Rule (RHR)

Right Hand Rule To determine direction of a series of 3D perpendicular vectors, we use the RHR. RHR Thumb points to direction of +q or I Fingers point to or curls to direction of magnetic field (B) Palm of hand points to direction of the magnetic force (F)

Magnetic Field Induced by a Current Carrying Wire Using conventional current, point thumb in the direction the current moves through the wire. Curl fingers around the wire to determine the direction of the magnetic field at any point. Palm always points to the middle indicating a centripetal force always toward the center of the wire.

Current Carrying Loop RHR to a current carrying wire shaped in a loop direction of the current produces a magnetic field up inside the loop.

Solenoids Coils of current carrying wire With each loop, the magnetic field increases. strong magnetic field inside, and a weak magnetic field outside the solenoid. a north and south pole at its end. used as electromagnets and switches North pole South pole

Magnetic Force Fmagnetic = qvBsinθ Charged particles moving through a magnetic field experience a force. Fmagnetic = qvBsinθ Variable Units Direction Fmagnetic Newtons (N) Palm q=charge Coulombs (c) Thumb v = velocity Meters/second (m/s) B= magnetic field strength Tesla (T) = Fingers θ = angle between velocity and magnetic field direction Degrees (˚) Only perpendicular components yield a magnetic force. (Cross product) __N__ = __N__ C * m/s A*m

Magnetic Force Charged particles must be moving to experience a force. Stationary charges do not experience a force. (Look at v in the equation) The force is strongest perpendicular to the field, weakest parallel to the field. (Look at θ in the equation) If the charge is negative(-), flip the direction of the force 180˚ e- As a charge moves through the magnetic field, the force perpendicular to the motion of the charge causes the charge to move in a circular path. v F

Magnetic Force Fmagnetic = IlBsinθ Current carrying wires create their own magnetic field or can be placed in a magnetic field to experience a force. Fmagnetic = IlBsinθ Variable Units Direction Fmagnetic Newtons (N) Palm I=Current Amps (A) Thumb (conventional current) l = length of wire in field Meters (m) B= magnetic field strength Tesla (T) = Fingers θ = angle between velocity and magnetic field direction Degrees (˚) Only perpendicular components yield a magnetic force. (Cross product) __N__ = __N__ C * m/s A*m

Parallel Current Carrying Wires Current carrying wires produce their own magnetic force. Two parallel wire placed next to each other will experience a force on each other. Parallel wires with current going in the same direction have an attractive force. Parallel wires with current going in the opposite directions have a repulsive force.

To determine attraction or repulsion, place your thumb in the direction of the current in one wire. Point your fingers in the direction of the magnetic field from the second wire in between the two wires. If the force is toward the second wire it is attraction. If the force is away from the second wire it is repulsion.

Loudspeaker A loudspeaker uses magnets to create sound waves. A permanent magnet is placed behind a coil of wire. AC current moves through the solenoid causing changing magnetic poles. The current changes to the rate of the sound signal. The speaker cone then attracts and repels to the same beat creating different air densities which are the sound waves.

Homework Chapter 21 1-4, 6-15, 18 Chapter 21 20, 22-26, 30-37